In the mid to late Fall/early Winter the seas are very active...raging at
times. The crossing to the North can be trying for people who tend towards
seasickness. The water tends toward a greener cast than usual.

The big animals tend to be in evidence during late summer/early fall. This
is prime Whale Sharking time...though the water is not at it's best vis or
best conditions. But for the big boys this is the best time.

El Nino years are not great in Galapagos, the water becomes too warm for
pelagic schools to be near the surface (or at depths divers visit).

My best visits have been in February, as an overall experience, but if you
are looking for Whale Sharks they are almost never there that time of year.
In this early part of the year the shark schools are active and large and
the water is at it's best vis of any other time, the water is also tends to
be at it's calmest. The tuna, jack, barracuda etc schools are also, for some
reason, at their largest this time of year. To me this is the best time for
Galapagos diving....but, again, there will be no Whale Sharks...or if there
is it will be a special occasion :^)